docs: def hkdf_expand(salt, info, shared_secret):
      """Derive encryption keys from shared secret."""
      from cryptography.hazmat.primitives import hashes
      from cryptography.hazmat.primitives.kdf.hkdf import HKDF
      from cryptography.hazmat.backends import default_backend
      hkdf = HKDF(
          algorithm=hashes.SHA512(),
          length=32,
          salt=salt.encode(),
          info=info.encode(),
          backend=default_backend()
      )
      return hkdf.derive(shared_secret)